Nice people do racism too. Liberal commitment to a multi-ethnic Britain is wilting. Some very nice folk have apparently decided that the nation's real problem is too many immigrants of too many kinds. Faced with a daily onslaught against migrants it may be understandable to give in to populist bigotry; but it is not forgivable.

Take this, for example: "National citizenship is inherently exclusionary." So no foreigners need ever apply for naturalisation, then. And " ... public anxiety about migration ... is usually based on a rational understanding of the value of British citizenship and its incompatibility with over-porous borders". Straight from the lexicon of the far right. And best of all: "You can have a [generous] welfare state provided that you are a homogenous society with intensely shared values."

Is this the wit and wisdom of Enoch Powell? Jottings from the BNP leader's weblog? Actually they are extracts from an article in the Observer, penned by the liberal intellectual David Goodhart, who I have always suspected is too brainy for his own good. He is just one of several liberal thinkers now vigorously making what they consider a progressive argument against immigration. It goes like this: the more diverse a society, the less likely its citizens are to share common values; the fewer common values, the weaker the support for vital institutions of social solidarity, such as the welfare state and the National Health Service.

There are perfectly good reasons to worry about how we respond to immigration, not least the downward pressure on workers' wages; the growth of racial inequality; and the exploitation of illegals exposed by the Morecambe Bay tragedy. But as Polly Toynbee elegantly pointed out in these pages last week, the answer to these problems is not genteel xenophobia, but trade union rights, backed by equality and employment law.

The xenophobes should come clean. Their argument is not about immigration at all. They are liberal Powellites; what really bothers them is race and culture. If today's immigrants were white people from the old Commonwealth, Goodhart and his friends would say that they pose no threat because they share Anglo-Saxon values. They may not even object to Anglophile Indians - as long as they aren't Muslims.

Unfortunately for liberal Powellites, the real history of the NHS shatters their fundamental case against diversity. The NHS is a world-beating example of the way that ethnic diversity can create social solidarity. Launched by a Welshman, built by Irish labourers, founded on the skills of Caribbean nurses and Indian doctors, it is now being rescued by an emergency injection of Filipino nurses, refugee ancillaries and antipodean medics. And it remains 100% British.

Virtually all of our public services have depended heavily on immigrants. Enoch Powell was forced to admit as much when, as minister for health he advertised for staff in the Caribbean. His new admirers will discover that a rapidly depopulating Europe will have no choice but to embrace diversity.

For the moment, however, the liberal Powellites are gaining support in high places. Their ideas are inspired by the work of the American sociologist Robert Putnam, a Downing Street favourite. He purports to show that dynamic, diverse communities are more fragmented than stable, monoethnic ones. But the policy wonks have forgotten that Putnam's research was conducted in a society so marked by segregation that even black millionaires still live in gated ghettoes.

The prime minister still seems uneasy on the issue. Last week, he wavered uncertainly between backing his robustly pro-immigration home secretary, and a desperately defensive response to Michael Howard's goading that the government was in a mess on the topic.

Oddly enough, this is a place in the arena of world politics where the PM does not stand shoulder to shoulder with George Bush. The Spanish-speaking former governor of Texas recently announced that he would "regularise" the status of millions of illegal Mexican immigrants who had slipped across the border to work. It's the kind of massive amnesty that would send the Daily Express into conniptions.

Even more peculiar, the prime minister appears to be ignoring not only David Blunkett but also his new best friend, the Labour mayor of London, Ken Livingstone, and Scotland's first minister, Jack McConnell. London wants more immigrants to keep pace with its booming economy, Scotland wants them to boost its ageing workforce.

Yet the liberal Powellites still seem prepared to confront a Bush-Blunkett-Livingstone-McConnell axis, because they are scared witless by the far right. They hope that by appeasing racism, they'll make it go away. But this is a beast with an insatiable appetite.

The French discovered that too late; the thuggish National Front is now France's second largest party, with one in five likely to vote for them in upcoming local elections. Liberal secularists who joined in the assault on the rights of French Muslims now have to find a convincing explanation for their cowardice, which has also betrayed the freedom of expression of French Jews and Christians.

In Holland, this spinelessness has ended up as straight leftwing racism. The previously liberal Dutch establishment is now pushing an asylum policy so extreme even the Sun was moved to criticise it.

The line up that favours managed migration and diversity - Blunkett, McConnell, Livingstone, Bush and the Sun - share one quality that the PM should envy more than any other at present: they are all popular with the public. Maybe the government ought to pay more heed to this focus group than the ones that see scary foreigners on every street corner.

Perhaps we should also be creating an even more progressive immigration policy, for example offering easier admission to those who will bring their skills to the depopulated regions of the north of England and Scotland. The Americans will next year offer more work permits to IT whizzkids from India than ever before; and before the middle of the century, the world's strongest economy will become its most ethnically diverse. Our own population is still over 92% white; we shouldn't be duped by anxious faint-hearts into becoming an all-white backwater.

There's one last reason the government ought to be suspicious about the advice of liberal Powellites. Minority Britons once looked to them for support. We learned the hard way that they are always totally committed to your cause - until they change their minds. In the immortal words of David Brent: "You have to get 100% behind someone before you can stab them in the back."

· Trevor Phillips is a journalist and broadcaster; since March 2003 he has been chairman of the Commission for Racial Equality

· David Goodhart's essay "Too diverse?" can be read in this month's issue of Prospect